14:00 〜 14:15
[SCG45-02] Mechanism of Sedex Mineralization and Sulfide Remobilization integrating Textures, Geochemistry, S and B stable Isotopes—Case Study from Kayad Zn-Pb Deposit, Western India
キーワード：Sedex, Multiple sulfur isotope, Boron isotope, Geochemistry, Critical metal, Ore Geology
The Kayad Zn-Pb deposit located in the Proterozoic Aravalli-Delhi fold belt in western India constitutes an integral part of the highest lead and zinc-producing metallogenic province in the Indian subcontinent. The ore mineralization, primarily represented by sphalerite and galena with appreciable amounts of pyrrhotite and chalcopyrite, occurs in the form of disseminations in quartzite, laminations and massive ores in graphite-bearing quartz mica schist, and in later intruding pegmatite/quartz veins. The laminated ores align with the schistosity whereas the massive ores disrupt the metamorphic fabric and accumulate at hinges of regional synformal fold. The massive ores are characterized by durchbewegung textures, low dihedral angles among ore mineral pairs, and ubiquitous sulfosalts such as pyrargyrite, gudmundite, tetrahedrite and breithauptite that occur as inclusions or complex intergrowths with sulfides. Trace element geochemistry further indicate higher concentrations of low melting chalcophile elements (LMCEs; Ag, Sb, Bi, As and Se in galena) in the massive ores compared to the disseminated and laminated ores. The vein-hosted Zn-Pb and Fe-Cu ores are associated with pervasive hydrothermal alteration (K±Na±Fe and Ca±Na) in the form of secondary albite, orthoclase, biotite, allanite, chamosite. The laminated and massive ores are conspicuous by the absence of such alteration. Ti-in-biotite thermometer yields a temperature of 500-650°C consistent with the known amphibolite facies metamorphism of the host rocks.
In-situ multiple sulfur isotope systematics of all sulfides show consistently positive d34S values irrespective of mineral species, mode of occurrence and texture, and fall within +2.7 and 11.3 ‰. Average D33S and D36S of pyrrhotite and chalcopyrite are -0.01±0.06‰ (2 S.D.) and 0.03±0.02‰ (2 S.D.) respectively with some outlier values up to 0.4‰. Texturally three types of tourmaline are identified: fine grains associated with disseminated and laminated ores, medium grains associated with clots of sulfides and large crystals in pegmatites. Based on X-site vacancy, all tourmaline types belong to alkali group and based on Y-site occupancy, the first two types belong to dravite sub-group whereas the pegmatitic tourmaline belong to schorl sub-group. The d11B compositions of tourmaline of different types are comparable to one another and furnish highly negative values ranging from -10.7‰ to -15.2‰.
This study proposes that the disseminated and laminated Zn-Pb ores at Kayad belong to SEDEX type whereas the massive Zn-Pb ores formed via sulfide partial melting and plastic flow of the pre-existing SEDEX mineralization during regional metamorphism, as evidenced by melt textures, plenitude of LMCEs, peak metamorphic temperature and indistinguishable d34S values between laminated SEDEX ore and massive sulfide ore. It also demonstrates that this form of hybrid mobilization is highly efficient in concentrating critical metals to economic grades. The positive d34S of the laminated/disseminated SEDEX sulfides, and the dravitic composition and negative d11B of co-genetic tourmaline can be best explained by fluid derivation from continental evaporite contrary to the general model for SEDEX mineralization. In contrast, the vein-hosted Fe-Cu sulfides likely formed from granitic hydrothermal fluids, which also aided in the remobilization of the vein-hosted Zn-Pb sulfides, as evidenced by alteration types, schorl to dravitic composition and negative d11B values of tourmaline. Sulfur was probably redistributed from the SEDEX ores during remobilization as indicated by overlapping isotopic compositions
In-situ multiple sulfur isotope systematics of all sulfides show consistently positive d34S values irrespective of mineral species, mode of occurrence and texture, and fall within +2.7 and 11.3 ‰. Average D33S and D36S of pyrrhotite and chalcopyrite are -0.01±0.06‰ (2 S.D.) and 0.03±0.02‰ (2 S.D.) respectively with some outlier values up to 0.4‰. Texturally three types of tourmaline are identified: fine grains associated with disseminated and laminated ores, medium grains associated with clots of sulfides and large crystals in pegmatites. Based on X-site vacancy, all tourmaline types belong to alkali group and based on Y-site occupancy, the first two types belong to dravite sub-group whereas the pegmatitic tourmaline belong to schorl sub-group. The d11B compositions of tourmaline of different types are comparable to one another and furnish highly negative values ranging from -10.7‰ to -15.2‰.
This study proposes that the disseminated and laminated Zn-Pb ores at Kayad belong to SEDEX type whereas the massive Zn-Pb ores formed via sulfide partial melting and plastic flow of the pre-existing SEDEX mineralization during regional metamorphism, as evidenced by melt textures, plenitude of LMCEs, peak metamorphic temperature and indistinguishable d34S values between laminated SEDEX ore and massive sulfide ore. It also demonstrates that this form of hybrid mobilization is highly efficient in concentrating critical metals to economic grades. The positive d34S of the laminated/disseminated SEDEX sulfides, and the dravitic composition and negative d11B of co-genetic tourmaline can be best explained by fluid derivation from continental evaporite contrary to the general model for SEDEX mineralization. In contrast, the vein-hosted Fe-Cu sulfides likely formed from granitic hydrothermal fluids, which also aided in the remobilization of the vein-hosted Zn-Pb sulfides, as evidenced by alteration types, schorl to dravitic composition and negative d11B values of tourmaline. Sulfur was probably redistributed from the SEDEX ores during remobilization as indicated by overlapping isotopic compositions